如何在用Flutter webview显示网页时自动登录？

import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';
import 'package:shared_preferences/shared_preferences.dart';

class AutoLoginWebView extends StatefulWidget {
  @override
  _AutoLoginWebViewState createState() => _AutoLoginWebViewState();
}

class _AutoLoginWebViewState extends State<AutoLoginWebView> {
  final Completer<WebViewController> _controller =
      Completer<WebViewController>();
  bool _isLoading = true;

  @override
  void initState() {
    super.initState();
    _autoLogin();
  }

  Future<void> _autoLogin() async {
    // 获取用户凭证
    SharedPreferences prefs = await SharedPreferences.getInstance();
    String username = prefs.getString('username');
    String password = prefs.getString('password');

    // 加载WebView
    setState(() {
      _isLoading = true;
    });

    // 构建登录URL
    String loginUrl = 'https://example.com/login?username=$username&password=$password';

    // 注入JavaScript执行自动登录
    String autoLoginScript = '''
      document.getElementById('username').value = '$username';
      document.getElementById('password').value = '$password';
      document.getElementById('loginForm').submit();
    ''';

    // 加载WebView并执行自动登录
    await _controller.future.then((controller) {
      controller.loadUrl(loginUrl);
      controller.evaluateJavascript(autoLoginScript);
    });

    // 登录完成后隐藏加载状态
    setState(() {
      _isLoading = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Auto Login WebView'),
      ),
      body: Stack(
        children: [
          WebView(
            initialUrl: 'https://example.com',
            javascriptMode: JavascriptMode.unrestricted,
            onWebViewCreated: (WebViewController webViewController) {
              _controller.complete(webViewController);
            },
            onPageFinished: (String url) {
              setState(() {
                _isLoading = false;
              });
            },
          ),
          if (_isLoading)
            Center(
              child: CircularProgressIndicator(),
            ),
        ],
      ),
    );
  }
}